[FR] Add support to modify the colour palettes used when using an SF symbol for an icon

[philipross]

Currently when using any SF symbol as an icon, the colour palette used is the accent colour

Examples: symbol:timer.circle.fill shows an icon rendered like this:

However, the SF Symbols app allows quite a bit of customisation for the look of an SF symbol. The same symbol with a slightly different colour palette is shown below:

This change will allow orgs to more closely align the colours used to their organisation's branding colours.